1. Field of the Invention
The present invention relates to a serial type recording apparatus for recording characters and images on a recording medium while scanning a recording head against the recording medium. More particularly, the present invention relates to a shuttle type recording apparatus in which a plurality of recording heads arranged at a predetermined interval can record divided recording areas for the respective recording heads.
The present invention is particularly suitable for a recording apparatus for recording by applying coloring agent on the recording medium in accordance with image data, and it is more particularly suitable for an ink jet type recording apparatus in which liquid recording ink is discharged as the coloring agent to make a record.
The present invention is applicable to any equipment which uses the recording medium such as paper, cloth, skin, unwoven cloth or OHP sheet and further metal or the like. Specific application equipments include office products such as a printer, a copying machine and a facsimile machine and industrial manufacturing machines.
2. Related Background Art
The serial type recording apparatus in which recording is made while scanning the recording head, has been commonly used in various recording apparatus because it is less expensive than a recording apparatus in which recording is made by using a full line head which covers an entire width of the recording medium such as recording sheet or the like.
In the serial type recording apparatus, a material which reacts to a heat generation element of a thermal head to a dedicated thermal sheet and a material which cause a dedicated photo-sensitive sheet to optically generates a recording color have been known as the material to generate color of the coloring agent to the recording material. As a system to make a record by applying the coloring agent to the recording material by the recording head, various systems have been put into practice and proposed. For example, an impact recording system in which an ink ribbon having liquid ink impregnated as the coloring agent is pressed and abutted against the recording medium by a print wire to transfer the ink, a thermal fusion transfer recording system or a thermal sublimation system in which a heat generating element of a thermal head is reacted to an ink ribbon head having a solid coloring agent applied to transfer the ink and an ink jet system in which liquid recording ink is discharged to make a record.
Recently, from a stand point of plain sheet recording, the latter recording system for applying the coloring agent is main stream. Among them, the ink jet recording system has advantages of low noise, low running cost, easiness to make the apparatus compact, ability of plain sheet recording and easiness for color recording, and has been commonly used in the recording apparatus such as a printer and a copying machine.
In the serial type recording apparatus, recording heads each of which allows the recording only in a relatively small limited area of the recording element such as a discharge port provided in the recording head are arranged on a carriage and they are sequentially scanned to make a record. Thus, it is relatively difficult to increase a recording speed and the increase of the recording speed has been a problem for the serial type.
On the other hand, in order to increase the speed of the image recording, it has been proposed and put into practice to increase a recording width (arrangement range of recording elements) of the recording head, to increase the carriage speed and a recording frequency to reduce a scan time or to scan bilaterally to make a record. However, each system has limitation. For example, in order to increase the recording width, corresponding improvement of precision in manufacturing the head is required and the recording head becomes expensive and a capacity of print buffer for temporarily storing the record data increases so that a problem is raised in terms of cost. In the system in which the color is generated by utilizing heat or the coloring agent is applied, means for preventing the deterioration of the recording quality or the break of the head due to self-temperature-rise of the recording head is needed particularly when the recording width is large. In the ink jet recording system in which the liquid recording ink which is not in contact with the recording medium, when a recording head of a large recording width, means to prevent the deterioration of the recording quality due to cockling of the recording medium by absorption of moisture of the ink is complex. When the recording frequency is raised, it is necessary to increase the scan speed of the carriage to maintain a certain pixel density, but in this case, a load of a drive source increases and the recording quality may be deteriorated by vibration of the ink in the recording head due to the high speed of the carriage.
A system which is relatively effective to increase the speed of the serial type image recording apparatus is disclosed in JP-A-50-81437 and U.S. Pat. No. 4,272,771. This reference discloses that, in order to concurrently print on a left half and a right half of a print line, a left print head assembly and a right print head assembly supported by one carriage mechanism are used to attain the speed-up of approximately two times. It also teaches that a higher recording speed may be attained by increasing the number of print head assemblies to more than two or conducting the bilateral printing.
However, in the construction for recording on one sheet by a plurality of recording heads as is the case of the prior art, a density difference may appear between recording areas recorded by the respective heads because of a difference in characteristics of the heads or a difference in characteristics of recording materials such as inks and ink ribbons.
FIGS. 1A and 1B show a phenomenon thereof. Here, two heads, head A and head B are used in which the head B has an output characteristic of a higher density than the head A. FIG. 1A shows record results for three stages of print duty, 25%, 50% and 100% when the recording areas of the head A and the head B is perfectly divided into two parts at the center of the sheet. As seen from FIG. 1A, the density difference can be clearly observed at the boundary for every duty.
On the other hand, FIG. 1B shows similar print results when certain overlapping area is provided in the recording areas of the head A and the head B. For the overlapping recording area, substantially half of the data of each of the two heads is thinned and the image is complementarily completed by overlapping the records by the both heads. Accordingly, the overall density of the overlapping recording area is higher than that of the recording area by only the head A, and lower than that of the recording area by only the head B. In this case, the density difference on the both sides of the overlapping area is clear although it does not appear as clearly as it is in FIG. 1A. In this manner, the problem of the density difference due to the difference of the output characteristics between the heads has not been completely solved when a plurality of recording heads are used and the recording is made by dividing the recording area. Accordingly, the above prior art recording apparatus cannot provide a complete image.